The invention relates to structures and compositions for the efficient storage of heat or cold by controlling the absorption or emission of thermal energy by certain salt hydrates at their melting point.
There is a great deal of interest in phase change thermal energy storage systems at the present time due in part to the ability of these materials to store large amounts of heat or cold by means of their latent heat of fusion and heat of crystallization exhibited during phase changes of these materials.
It is well known that the latent heat capacity of a material during melting is much greater than the specific heat capacity per degree due to a temperature change of either the solid heat storage material prior to melting or the resulting liquid after all of the material has melted. Heat is evolved when the melted salts are cooled through their melting point and the heat energy from the heat of crystallization is recovered. Heat is absorbed when the crystallized salts are heated through their melting point and their heat of fusion is absorbed.
Salt hydrates tend to supercool, i.e., they sometimes remain liquid when the melted salt hydrate is cooled below its melting temperature. The amount of heat recovered in that circumstance is equal to the specific heat capacity per degree, i.e., the energy emitted when the temperature of the supercooled liquid is lowered and does not include the heat of crystallization. Nucleating agents, which induce crystallization, have been described to avoid this problem. Borax, otherwise known as sodium tetraborate decahydrate, is described in U.S. Pat. No. 2,677,664 issued May 4, 1954, to Telkes as a suitable nucleating agent for use with sodium sulfate decahydrate.
Another problem with many salt hydrates, particularly those based on inexpecsive Glauber's salt (sodium sulfate decahydrate), is that when they melt and thereby undergo phase transition, a liquid/solid mixture is formed, which separates into a heterogeneous mixture. For example, sodium sulfate decahydrate, after repeated heating and cooling, develops three distinct phases involving anhydrous sodium sulfate in one layer, sodium sulfate decahydrate in another layer, and an aqueous solution of sodium sulfate in a third layer. This leads to inefficiency in the heat or cold storage system, because the largest heat or cold storage capacity is obtained from the decahydrate, which does not completely re-form from the heterogeneous mixture.
Telkes, in U.S. Pat. No. 3,986,969, issued Oct. 19, 1976, describes a method for preventing the phase separation and supercooling of sodium sulfate decahydrate by mixing the salt hydrate and a nucleating agent together, then suspending them in a thixotropic mixture. In this highly divided form, it is believed that the problem caused by phase separation is minimized. However, thixotropic mixtures will eventually separate on heat cycling.
Laing, in U.S. Pat. No. 3,720,198, issued Mar. 13, 1973, described heat storage substances that are dispersed with the aid of "body forming" materials which are fused in place. An intergal aspect of that process involves the uniform dispersion of a nucleating agent such as borax in the heat storage substance throughout the material. These materials are formed by mixing the nucleating agent with the heat storage substance and the "body forming" material, pressing the materials into the desired form and then sintering, but are impractical or impossible to manufacture or fail to prevent separation after repeated heat and cold cycling.